Pseudomonas aeruginosa is an important opportunistic pathogen both in terms of the morbidity and mortality of infections it causes. Most patients with cystic fibrosis (CF), are colonized at an early age with this organism and most CF patients ultimately succumb to a chronic lung infection from P. aeruginosa. The reason for the extraordinary pathogenicity of P. aeruginosa in these patients, as compared to other Pseudomonads for example, is not clear. It is highly probable that the myriad of virulence determinants P. aeruginosa produces contributes to its pathogenic potential. Unfortunately, the exact contribution of these factors, alone or in combination, to even the simplest kind of P. aeruginosa infection has not yet been elucidated. In the past few years studies using molecular, biochemical and genetic approaches have begun to elucidate the structure-function relationships and mechanisms of regulation of virulence determinants. This research is directed at understanding the role of phospholipase C (PLC) production in the pathogenesis of P. aeruginosa infections. PLC has become recognized in recent years as a critical enzyme in both eukaryotic and prokaryotic biology. In eukaryotic organisms it is a critical second messenger in cellular processes, particularly in the function of specific and nonspecific immune mechanisms. In prokaryotic organisms there has been a resurgence of interest in PLC as a critical virulence determinant, both in gram negative and gram positive infections. P. aeruginosa produces two distinct PLCs that could play a significant role in the pathogenesis of lung, as well a other kinds of infections. One PLC is cytolytic (PLC-H) on human erythrocytes and neutrophils, while the other is not (PLC-N) lytic to these kind of cells. These and other features suggest structure-functions relationships between PLC activity and cytolytic activity that will be investigated in this research project. A more complete understanding of the structure-function relationships of both PLCs will lead to better understanding of their role in the pathogenesis of P. aeruginosa, and could result in therapeutic interventions for P. aeruginosa lung infections that were not previously considered. We also propose that derivatives of the substrate products produced by the action of both PLCs on phosphatidylcholine, the major essential lipid in lung surfactant, significantly contribute to the pathogenesis of P. aeruginosa infections. We hypothesize that some of these derivatives are especially relevant to the survival of this organism in the lungs of CF patients. We will investigate how a class of compounds, known as osmoprotectants including, glycine betaine, are able to induce the synthesis of both PLCs in P. aeruginosa. This compound, derived from the one of substrate products of both PLCs, can provide for the survival of this organism in a high osmotic environment, such as found in the lungs of CF patients or in the urinary tract. We propose that understanding this unusual regulatory process could lead to the discovery of novel agents which might at least temper the pathogenic potential of P. aeruginosa, if not directly affect its ability to persist in the lungs of CF patients, or survive in the high osmotic environment of the urinary tract.